1 . Field of the Invention
The present invention relates to a silicon wafer and a method of manufacturing the silicon wafer, and more particularly a silicon wafer which has a film formed by chemical vapor deposition (a CVD film), particularly a film formed by plasma chemical vapor deposition (a plasma CVD film), serving as both a gettering layer and a protective film for preventing autodoping. Further, the present invention relates to a silicon epitaxial wafer produced from the silicon wafer.
2 . Description of the Related Art
Of the two main faces of a silicon wafer other than its edge, a device is fabricated on one main face (hereinafter referred to as a "front surface") of the silicon wafer. The technique of forming a thin film on the other main face (hereinafter referred to as a "back surface") of the silicon wafer has two principal objects.
One object is to utilize the thin film as a layer for gettering impurities such as heavy metals, and the other object is to utilize the thin film as a protective film for preventing autodoping that would otherwise occur when a layer is epitaxially grown on the silicon wafer.
Gettering is the technique of collecting impurities, such as heavy metals, produced during the course of manufacture of a silicon electronic device on the silicon wafer outside the regions in the vicinity of the front surface of the silicon wafer where devices are fabricated. By virtue of this technique, there can be prevented the degradation of the characteristics of the devices by impurities such as heavy metals, which in turn allows an increase in the ratio of non-defective silicon devices to the entire silicon devices to be manufactured.
As disclosed in, e.g., Japanese Patent Application Laid-open No. 59-186331, one representative gettering technique is the technique of forming a gettering layer by depositing polycrystalline silicon on the back surface of the silicon wafer through use of low-pressure chemical vapor deposition (the low-pressure CVD method). In order to improve the gettering capability, there has been proposed a method of depositing amorphous silicon in place of the polycrystalline silicon (see Japanese Patent Application Laid-open No. 4-2133).
Autodoping is a phenomenon in which a dopant evaporates principally from the back surface of the silicon wafer into the gas phase to be used for epitaxial growth when a silicon epitaxial wafer is manufactured, and the thus-evaporated dopant is incorporated into the epitaxial layer.
In a case where the resistivity of the silicon wafer to be subjected to epitaxial growth is low, or where there is a high concentration of dopants, autodoping occurs to a noticeable extent. If the epitaxial layer containing dopants in small concentration is grown, a transition width (i.e., the width of an area where the concentration of dopants changes in the vicinity of the interface between an epitaxial layer and the silicon wafer) is extended, which makes it difficult to control the resistivity of the epitaxial layer. If a silicon device is fabricated through use of a silicon wafer having an epitaxial layer with a wide transition area, the device fails to exhibit its originally-designed characteristics, thereby rendering the device defective.
For this reason, in a commonly-used technique, a protective film for the purpose of preventing autodoping is formed on the back surface of a silicon wafer in order to prevent the autodoping in a case where the silicon wafer having a high concentration of dopants is subjected to epitaxial growth. An atmospheric-pressure silicon oxide film (an atmospheric-pressure CVD oxide film) grown in an atmospheric-pressure chemical vapor deposition apparatus (an atmospheric-pressure CVD system) is primarily used as the protective film.
As the integration and performance of the silicon devices have been improving in recent years, there has arisen a demand for improvements in persistency of gettering capability of the silicon wafer to be used as a material of the silicon electronic devices, in addition to a demand for improvements in the gettering capability.
However, if the gettering layer is formed by depositing the polycrystalline silicon or amorphous silicon, there are only a limited number of elements which can be expected to be removed by gettering. For this reason, there is a limit to the gettering capability of the gettering layer. As the silicon wafer receives heat during the fabrication of the device, transformation of the polycrystalline silicon or amorphous silicon into monocrystalline silicon proceeds, thereby resulting in deterioration of the gettering capability. Thus, there is a problem with the persistency of the gettering capability of the silicon wafer.
In a case where a silicon monocrystal is epitaxially grown on the front surface of the silicon wafer having a gettering layer of polycrystalline or amorphous silicon on the back surface, dopants easily penetrate through the polycrystalline or amorphous silicon. Particularly, in a case where the silicon wafer to be subjected to epitaxial growth contains a high concentration of dopants, polycrystalline or amorphous silicon is deposited on the back surface of the silicon wafer, and an atmospheric-pressure CVD oxide film is further deposited on the polycrystalline or amorphous silicon, thereby preventing autodoping. These deposition operations are carried out in individual steps, so that the production cost increases.